Electrode for secondary battery and secondary battery

ABSTRACT

An electrode for a secondary battery includes a substrate and an active material layer. The active material layer is placed on a surface of the substrate. In a surface of the active material layer, one or more grooves are formed. The groove extends linearly in a direction perpendicular to a thickness direction of the active material layer. The groove has an open portion on a periphery of the active material layer. The open portion opens in the direction perpendicular to the thickness direction. The groove includes a first region and a second region. The second region is interposed between the open portion and the first region. In a cross section perpendicular to a direction in which the groove extends, the first region has a first cross-sectional area, and the second region has a second cross-sectional area. The second cross-sectional area is smaller than the first cross-sectional area.

This nonprovisional application is based on Japanese Patent ApplicationNo. 2021-153017 filed on Sep. 21, 2021, with the Japan Patent Office,the entire contents of which are hereby incorporated by reference.

BACKGROUND Field

The present disclosure relates to an electrode for a secondary batteryand a secondary battery.

Description of the Background Art

Japanese Patent Laying-Open No. 2021-009846 discloses a technique forenhancing electrolytic solution impregnation force and gas dischargeforce by embodying a patterned adhesive force on a surface of aseparation film.

SUMMARY

A “secondary battery” refers to a battery capable of charge anddischarge.

Hereinafter, a secondary battery may be simply called “a battery”.Generally, a battery includes an electrode and an electrolyte solution.The electrode includes an active material layer. The active materiallayer is porous. The electrolyte solution permeates the active materiallayer.

When permeation of the electrolyte solution into the active materiallayer is insufficient, inconvenient phenomena such as a decrease incycling performance, for example, are expected to occur. To address thisproblem, a groove (a linear depressed portion) may be formed in asurface of the active material layer, for example. The groove may serveas a channel for an electrolyte solution. Forming the groove is expectedto facilitate permeation of the electrolyte solution.

The active material layer expands and shrinks due to charge anddischarge. While the active material layer expands, the electrolytesolution is ejected from the active material layer. With a groove formedin the active material layer, ejection of the electrolyte solution maybe facilitated. If ejection of the electrolyte solution is facilitated,the electrolyte solution in the active material layer may becomeexhausted. As a result, cycling performance may decrease, contrary tothe original intention.

An object of the present disclosure is to provide an electrode whichenables easy permeation of an electrolyte solution but does not enableeasy ejection thereof.

Hereinafter, the technical configuration and effects of the presentdisclosure will be described. It should be noted that the actionmechanism according to the present specification includes presumption.The action mechanism does not limit the technical scope of the presentdisclosure.

1. An electrode for a secondary battery includes a substrate and anactive material layer. The active material layer is placed on a surfaceof the substrate. In a surface of the active material layer, one or moregrooves are formed. The groove extends linearly in a directionperpendicular to a thickness direction of the active material layer. Thegroove has an open portion on a periphery of the active material layer.The open portion opens in the direction perpendicular to the thicknessdirection.

The groove includes a first region and a second region. The secondregion is interposed between the open portion and the first region. In across section perpendicular to a direction in which the groove extends,the first region has a first cross-sectional area, and the second regionhas a second cross-sectional area. The second cross-sectional area issmaller than the first cross-sectional area.

The groove extends linearly on a surface of the active material layer.The groove opens at a periphery of the active material layer. The openportion of the groove serves as an inlet/outlet for an electrolytesolution. Hereinafter, the direction in which the groove extends is alsocalled “an extending direction”. The cross-sectional area of the grooverefers to the area of a cross section perpendicular to the extendingdirection. Conventionally, the cross-sectional area of the groove doesnot change. In the present disclosure, the cross-sectional area of thegroove does change. That is, the groove includes a first region and asecond region. The second region is closer to the open portion (aninlet/outlet) than the first region is. The cross-sectional area of thesecond region is smaller than the cross-sectional area of the firstregion. The second region may function as a weir or a check valve.

The groove is a channel for an electrolyte solution. When theelectrolyte solution passes through the groove, pressure loss occurs.The groove can be regarded as a pipe to allow for estimating thepressure loss.

FIG. 1 is a view for describing pressure loss.

In the graphs in FIG. 1 , the vertical axis represents pressure loss,and the horizontal axis represents the pipe diameter. The pipe diametermay be regarded as the square root of the cross-sectional area of thegroove. For the sake of convenience, the square root of thecross-sectional area of the groove may also be called “the groovediameter”. The curve in the graph is derived by the Darcy-Weisbachequation “ΔP=λLρU²/(2d)”, where “ΔP” represents pressure loss, “λ”represents the pipe friction factor, “L” represents the groove length,“ρ” represents the electrolyte solution density, “U” represents theaverage flow speed, and “d” represents the groove diameter. When theactive material layer expands, the groove shrinks. The pressure lossduring expansion of the active material layer (during shrinkage of thegroove) is “ΔP_(exp)”. When the active material layer shrinks, thegroove enlarges. The pressure loss during shrinkage of the activematerial layer (during enlargement of the groove) is “ΔP_(con)”. “ϵ” isthe ratio of the volume of the active material layer during a shrunkperiod (during a discharged period) to the volume of the active materiallayer during an expanded period (during a charged period). “ϵ” maychange depending on, for example, the type of an active material. Forexample, “ϵ=0.9” may be satisfied. As the volume of the active materiallayer changes due to charge and discharge, the pressure loss of thegroove also changes. During expansion of the active material layer(during shrinkage of the groove), pressure loss increases. Duringshrinkage of the active material layer (during enlargement of thegroove), pressure loss decreases. The amount of change in pressure lossdue to charge and discharge is “ΔP_(exp)−ΔP_(con)”.

When the groove includes a first region and a second region, thepressure loss during expansion of the active material layer (duringshrinkage of the groove) is “ΔP′_(exp)”, and the pressure loss duringshrinkage of the active material layer (during enlargement of thegroove) is “ΔP′_(con)”. “ΔP′_(exp)” and “ΔP_(con)” are determined byassigning the length of the first region into “L₁”, the diameter of thefirst region into “d₁”, the average flow speed of the first region into“U₁”, the length of the second region into “L₂”, the diameter of thesecond region into “d₂”, and the average flow speed of the second regioninto “U₂”. In the present disclosure, “d_(2<)d₁” is satisfied. Theamount of change in pressure loss due to charge and discharge is“ΔP′_(exp)−ΔP′_(con)”.

When the second region with a smaller cross-sectional area is present,the amount of change in pressure loss due to charge and discharge,“ΔP′_(exp)−ΔP′_(con)”, may increase significantly. According to a novelfinding of the present disclosure, the greater the amount of change inpressure loss due to charge and discharge is, the more inhibited theejection of the electrolyte solution tends to be during expansion of theactive material layer.

Further, during shrinkage of the active material layer, the electrolytesolution may be sucked through the open portion into the active materiallayer. While the electrolyte solution is being sucked, the presence ofthe second region with a smaller cross-sectional area may significantlyfacilitate permeation of the electrolyte solution. It seems to beattributed to capillary action.

As a result of the above actions working synergistically, in the presentdisclosure, it is possible to provide an electrode which enables easypermeation of an electrolyte solution but does not enable easy ejectionthereof.

2. In the cross section perpendicular to a direction in which the grooveextends, the first region may have a first depth, and the second regionmay have a second depth. The second depth may be smaller than the firstdepth.

For example, the depth of each region may be changed so as to adjust thecross-sectional area of the region.

3. In the cross section perpendicular to a direction in which the grooveextends, the first region may have a first width, and the second regionmay have a second width. The second width may be smaller than the firstwidth.

For example, the width of each region may be changed so as to adjust thecross-sectional area of the region.

4. The groove may include two second regions. The first region may beinterposed between the two second regions.

For example, the second region may be connected to both ends of thefirst region. When the second region is connected to both ends of thefirst region, nonuniform distribution of the electrolyte solution in aplanar direction is expected to be reduced, for example.

5. The second region may be connected to the open portion.

When the second region is directly connected to the inlet/outlet, thefunction as a weir is expected to be enhanced.

6. The following Expressions 1 to 5 may be satisfied:

(ΔP′ _(exp) −ΔP′ _(con))/(ΔP _(exp) −ΔP _(con))>1   Expression 1

ΔP _(con)=λ₁(L ₁ +L ₂)U ²/(2d ₁)   Expression 2

ΔP _(exp)=λ₁(L ₁ +L ₂)U ₁ ²/(2ϵd ₁)   Expression 3

ΔP′ _(con)=λ₁ L ₁ U ₁ ²/(2d₁)+λ₂ L ₂ U ₂ ²/(2d ₂)   Expression 4

ΔP′ _(exp)=λ₁ L ₁ U ₁ ²/(2ϵd ₁)+λ₂ L ₂ U ₂ ²/(2ϵd ₂)   Expression 5

where

“λ₁” represents a pipe friction factor in the first region,

“λ₂” represents a pipe friction factor in the second region,

“L₁” represents a length of the first region in the direction in whichthe groove extends,

“L₂” represents a length of the second region in the direction in whichthe groove extends,

“U₁” represents an average flow speed in the first region,

“U₂” represents an average flow speed in the second region,

“d₁” represents a square root of the first cross-sectional area,

“d₂” represents a square root of the second cross-sectional area, and

“ϵ” represents a ratio of a volume of the active material layer during adischarged period to a volume of the active material layer during acharged period.

When the above Expression 1 is satisfied, it is expected that permeationof the electrolyte solution is facilitated and ejection of theelectrolyte solution is inhibited.

The above Expressions 2 to 5 are derived by the Darcy-Weisbach equation.Usually, the Darcy-Weisbach equation includes “fluid density (ρ)” (seeFIG. 1 ).

However, fluid density can be cancelled in the division in the aboveExpression 1, so in the above Expressions 2 to 5, “fluid density (ϵ)” isomitted.

7. The following Expression 6 may be further satisfied:

(ΔP′ _(exp) −ΔP′ _(con))/(ΔP _(exp) −ΔP _(con))≤5.5   Expression 6

When the above Expression 6 is satisfied, it is expected that permeationof the electrolyte solution is further facilitated and ejection of theelectrolyte solution is further inhibited.

8. A secondary battery includes the electrode according to the aboveitems 1 to 7 and an electrolyte solution.

The battery may exhibit an excellent cycling performance, for example.It may be because the electrolyte solution easily permeates into theelectrode and the electrolyte solution is not easily ejected from theelectrode.

The foregoing and other objects, features, aspects and advantages of thepresent disclosure will become more apparent from the following detaileddescription of the present disclosure when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for describing pressure loss.

FIG. 2 is a schematic view illustrating an electrode according to thepresent embodiment.

FIG. 3 is schematic cross-sectional views of an example of a firstregion and an example of a second region.

FIG. 4 is a schematic cross-sectional view illustrating a secondarybattery according to the present embodiment.

FIG. 5 presents top views of a groove of test electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Definitions of Terms, etc.

Next, an embodiment of the present disclosure (which may also be simplycalled “the present embodiment”) and an example of the presentdisclosure (which may also be simply called “the present example”) willbe described. It should be noted that neither the present embodiment northe present example limits the technical scope of the presentdisclosure.

Herein, expressions such as “comprise”, “include”, and “have”, and othersimilar expressions (such as “be composed of”, for example) areopen-ended expressions. In an open-ended expression, in addition to anessential component, an additional component may or may not be furtherincluded. The expression “consist of” is a closed-end expression.However, even when a closed-end expression is used, impurities presentunder ordinary circumstances as well as an additional element irrelevantto the technique according to the present disclosure are not excluded.The expression “consist essentially of” is a semiclosed-end expression.A semiclosed-end expression tolerates addition of an element that doesnot substantially affect the fundamental, novel features of thetechnique according to the present disclosure.

Herein, a singular form also includes its plural meaning, unlessotherwise specified.

Herein, expressions such as “may” and “can” are not intended to mean“must” (obligation) but rather mean “there is a possibility”(tolerance).

Herein, any geometric term (such as “parallel”, “vertical”, and“perpendicular”, for example) should not be interpreted solely in itsexact meaning. For example, “parallel” may mean a geometric state thatis deviated, to some extent, from exact “parallel”. Any geometric termherein may include tolerances and/or errors in terms of design,operation, production, and/or the like. The dimensional relationship ineach figure may not necessarily coincide with the actual dimensionalrelationship. The dimensional relationship (in length, width, thickness,and the like) in each figure may have been changed for the purpose ofassisting the understanding of the technique according to the presentdisclosure. Further, a part of a configuration may have been omitted.

Herein, a numerical range, such as “from m to n %”, for example,includes both the upper limit and the lower limit, unless otherwisespecified. That is, “from m to n %” means a numerical range of “not lessthan m % and not more than n %”. “Not less than m % and not more than n%” includes “more than m % and less than n %”. Moreover, any numericalvalue selected from a certain numerical range may be used as a new upperlimit or a new lower limit. For example, any numerical value from acertain numerical range may be combined with any numerical valuedescribed in another location of the present specification or in a tableor a drawing to set a new numerical range.

Herein, all numerical values are regarded as being modified by the term“about”. The term “about” may mean ±5%, ±3%, ±1%, and/or the like, forexample. Each numerical value may be an approximate value that can varydepending on the implementation configuration of the technique accordingto the present disclosure. Each numerical value may be expressed insignificant figures. Each measured value may be the average valueobtained from multiple measurements performed. The number ofmeasurements may be 3 or more, or may be 5 or more, or may be 10 ormore. Generally, the greater the number of measurements is, the morereliable the average value is expected to be. Each measured value may berounded off based on the number of the significant figures. Eachmeasured value may include an error occurring due to an identificationlimit of the measurement apparatus, for example.

Herein, “D50” is defined as a particle size in volume-based particlesize distribution at which cumulative frequency of particle sizesaccumulated from the small size side reaches 50%. The volume-basedparticle size distribution may be obtained by measurement with alaser-diffraction particle size distribution analyzer.

Herein, when a compound is represented by a stoichiometric compositionformula such as “LiCoO₂”, for example, this stoichiometric compositionformula is merely a typical example. Alternatively, the compositionratio may be non-stoichiometric. For example, when lithium cobalt oxideis represented as “LiCoO₂”, the composition ratio of lithium cobaltoxide is not limited to “Li/Co/O=1/1/2” but Li, Co, and O may beincluded in any composition ratio, unless otherwise specified. Further,doping with a trace element and/or substitution may also be tolerated.

In the present specification, “a lithium-ion battery” is described.However, a lithium-ion battery is merely an example of a secondarybattery. The present embodiment may be applied to any secondary battery.

“SOC (state of charge)” herein refers to the percentage of the chargedcapacity of an electrode at a point in time in question relative to thefull charge capacity of the electrode.

Electrode for Secondary Battery

FIG. 2 is a schematic view illustrating an electrode according to thepresent embodiment.

An electrode 100 is for a secondary battery. The secondary battery isdescribed below. Electrode 100 may be a positive electrode, or may be anegative electrode, or may be a bipolar electrode. Electrode 100 is insheet form. Electrode 100 includes a substrate 10 and an active materiallayer 20.

Substrate

Substrate 10 is a support for active material layer 20. Substrate 10 maybe in sheet form, or may be in mesh form, for example. Substrate 10 mayhave a belt-like planar shape, for example. Substrate 10 may beelectrically conductive. The substrate may function as a currentcollector. Part of substrate 10 may be exposed from active materiallayer 20. To the exposed part of substrate 10, a current-collectingmember and/or the like may be bonded, for example.

Substrate 10 may have any thickness. Substrate 10 may have a thicknessfrom 5 to 50 μm or may have a thickness from 5 to 20 μm, for example.

Substrate 10 may include a metal foil and/or the like, for example.Substrate 10 may include at least one selected from the group consistingof an aluminum (Al) foil, an Al alloy foil, a copper (Cu) foil, a Cualloy foil, a nickel (Ni) foil, a Ni alloy foil, a titanium (Ti) foil,and a Ti alloy foil, for example. When electrode 100 is a positiveelectrode, substrate 10 may include an Al foil and/or the like, forexample. When electrode 100 is a negative electrode, substrate 10 mayinclude a Cu foil and/or the like, for example.

Active Material Layer

Active material layer 20 is placed on a surface of substrate 10. Activematerial layer 20 may be placed on only one side, or on both sides, ofsubstrate 10. Active material layer 20 may have any thickness. Activematerial layer 20 may have a thickness from 5 to 1000 μm, or may have athickness from 10 to 500 μm, or may have a thickness from 50 to 250 μm,for example.

Groove

In a surface of active material layer 20, one or more grooves 25(depressed portions) are formed. Groove 25 may be formed by embossingand/or the like, for example. A single groove 25 may be formed, or aplurality of grooves 25 may be formed. The plurality of grooves 25 maybe formed as parallel lines, or may be formed in a grid pattern, forexample. The “parallel lines” refer to a group of lines that areparallel to each other. When a plurality of grooves 25 are formed, thepitch (the interval) between adjacent grooves 25 may be from 0.1 to 10mm, for example. Groove 25 may have a length from 1 to 5000 mm, or mayhave a length from 1 to 1000 mm, for example.

Groove 25 extends in a direction perpendicular to a thickness directionof active material layer 20 (namely, to the Z-axis direction). Theextending direction (the X-axis direction) in FIG. 2 is an examplethereof. As long as it is perpendicular to the thickness direction, theextending direction may be not limited. Groove 25 extends linearly.Groove 25 may be a straight line, or may be bent, or may be curved, ormay be wavy, for example. Groove 25 may be an unbranched line, or may bea branched line. A plurality of grooves 25 may merge together.

Groove 25 may run across the surface of active material layer 20, forexample. Groove 25 has an open portion 23 on a periphery of activematerial layer 20. Open portion 23 opens in the direction perpendicularto the thickness direction (in the X-axis direction in FIG. 2 ). Inother words, groove 25 opens in a side wall of active material layer 20.The side wall of active material layer 20 may be inclined. Open portion23 may serve as an inlet/outlet for an electrolyte solution. Groove 25may have a plurality of open portions 23. For example, a single groove25 may branch into multiple grooves to form a plurality of open portions23.

Groove 25 includes a first region 21 and a second region 22. Firstregion 21 may also be called “a main groove portion”, “a centralportion”, and/or the like, for example. Second region 22 may also becalled “a narrow portion”, “a sub-groove portion”, “an end portion”,and/or the like, for example. First region 21 is connected to secondregion 22. Second region 22 is interposed between open portion 23 andfirst region 21. Second region 22 may be provided on only one side offirst region 21, or may be provided on both sides of first region 21. Inother words, groove 25 may include two second regions 22. First region21 may be interposed between two second regions 22. When second regions22 are connected to both ends of first region 21, nonuniformdistribution of the electrolyte solution in a planar direction of activematerial layer 20 is expected to be reduced, for example. Second region22 may be connected to open portion 23. Second region 22 may be locatedapart from open portion 23. First region 21 has a first length 21L.Second region 22 has a second length 22L. Second length 22L may beshorter than first length 21L. For example, the ratio of second length22L to first length 21L may be from 0.01 to 0.5, or may be from 0.05 to0.5, or may be from 0.1 to 0.3, for example.

FIG. 3 is schematic cross-sectional views of an example of a firstregion and an example of a second region.

Illustrated in FIG. 3 are the A-A cross section in FIG. 2 and the B-Bcross section in FIG. 2 . Each cross section is perpendicular to theextending direction of groove 25. First region 21 and second region 22,independently, may have any cross-sectional profile. The bottom face ofeach of these regions may be flat. The bottom face of each of theseregions may be not flat. A side wall of each of these regions may beperpendicular to the surface of active material layer 20. A side wall ofeach of these regions may be inclined. The cross-sectional profile ofeach of these regions may be rectangular, or may be U-shaped, or may beV-shaped, for example.

First region 21 has a first cross-sectional area. Second region 22 has asecond cross-sectional area. The second cross-sectional area is smallerthan the first cross-sectional area. Thus, second region 22 is capableof functioning as a weir for the electrolyte solution. For example, theratio of the second cross-sectional area to the first cross-sectionalarea may be from 0.1 to 0.9, or may be from 0.3 to 0.7.

As long as one or more grooves 25 each including first region 21 andsecond region 22 are formed in the surface of active material layer 20,one or more other grooves having a uniform cross-sectional area (notillustrated), for example, may be further formed in the surface ofactive material layer 20.

As for respective regions, its cross-sectional area may not change, ormay change. For example, within first region 21, the depth of groove 25may continuously change in a slope manner. For example, within secondregion 22, the width of groove 25 may continuously change in a taperedmanner.

For example, there may be a difference in level between first region 21and second region 22. In other words, the cross-sectional area of groove25 may change stepwise between first region 21 and second region 22.

For example, there may be no difference in level between first region 21and second region 22. In other words, the cross-sectional area of groove25 may change continuously between first region 21 and second region 22.For example, in the XY plane in FIG. 2 , groove 25 may extend in atapered shape. In other words, groove 25 may become narrower toward openportion 23. Alternatively, in the XZ plane in FIG. 2 , groove 25 mayextend in a tapered shape. In other words, groove 25 may becomeshallower toward open portion 23. In the present embodiment, even whenthere is no difference in level, it is considered that first region 21having the first cross-sectional area and second region 22 having thesecond cross-sectional area are present.

First region 21 may have a first depth 21D. Second region 22 may have asecond depth 22D. Second depth 22D may be smaller than first depth 21D.The ratio of second depth 22D to first depth 21D may be from 0.1 to 0.9,or may be from 0.3 to 0.7, for example. First depth 21D may be from 10to 400 μm, or may be from 50 to 300 μm, or may be from 50 to 200 μm, ormay be from 50 to 150 μm, for example. When the depth is not uniform inthe target region, the depth at the deepest point is regarded as thedepth of the target region.

First region 21 may have a first width 21W. Second region 22 may have asecond width 22W. Second width 22W may be smaller than first width 21W.The ratio of second width 22W to first width 21W may be from 0.1 to 0.9,or may be from 0.3 to 0.7, for example. First width 21W may be from 10to 500 μm, or may be from 50 to 300 μm, or may be from 50 to 200 μm, ormay be from 50 to 150 μm, for example. The width is perpendicular to thedepth. When the width is not uniform in the target region, the largestwidth of the target region is regarded as the width of the targetregion.

Pressure Loss

Regarding electrode 100, the following Expressions 1 to 5 may besatisfied:

(ΔP′ _(exp) −ΔP′ _(con))/(ΔP _(exp) −ΔP _(con))>1   Expression 1

ΔP _(con)=λ₁(L ₁ +L ₂)U ²/(2d ₁)   Expression 2

ΔP _(exp)=λ₁(L ₁ +L ₂)U ₁ ²/(2ϵd ₁)   Expression 3

ΔP′ _(con)=λ₁ L ₁ U ₁ ²/(2d₁)+λ₂ L ₂ U ₂ ²/(2d ₂)   Expression 4

ΔP′ _(exp)=λ₁ L ₁ U ₁ ²/(2ϵd ₁)+λ₂ L ₂ U ₂ ²/(2ϵd ₂)   Expression 5

where

“λ₁” represents a pipe friction factor in first region 21,

“λ₂” represents a pipe friction factor in second region 22,

“L₁” represents a length of first region 21 in the extending direction,

“L₂” represents a length of second region 22 in the extending direction,

“U₁” represents an average flow speed in first region 21,

“U₂” represents an average flow speed in second region 22,

“d₁” represents a square root of the first cross-sectional area,

“d₂” represents a square root of the second cross-sectional area, and

“ϵ” represents a ratio of a volume of active material layer 20 during adischarged period (during a shrunk period) to a volume of activematerial layer 20 during a charged period (during an expanded period).

In the above Expressions 1 to 5, each of “ΔP_(con), ΔP_(exp), ΔP′_(con),ΔP′_(exp)” represents pressure loss. In the above Expression 1,“ΔP_(exp)−ΔP_(con)” represents the amount of change in pressure loss dueto charge and discharge on the assumption that groove 25 consists offirst region 21. In the above Expression 1, “ΔP′_(exp)−ΔP′_(con)”represents the amount of change in pressure loss due to charge anddischarge when groove 25 includes first region 21 and second region 22.When the left side of the above Expression 1 is greater than 1, it isexpected that permeation of the electrolyte solution is facilitated andejection of the electrolyte solution is inhibited. The greater the leftside of the above Expression 1 is, the more facilitated the permeationof the electrolyte solution may be and the more inhibited the ejectionof the electrolyte solution may be.

For example, the following Expression 6 may be satisfied.

(ΔP′ _(exp) −P′ _(con))/(ΔP _(exp) −ΔP _(con))≥5.5   Expression 6

The left side of Expression 6 may be 6.4 or more, or may be 19 or more,for example. The left side of Expression 6 may be from 5.5 to 19, or maybe from 6.4 to 19, or may be from 5.5 to 6.4, for example.

“d₁” may be first depth 21D, or may be first width 21W, for example.“d₂” may be second depth 22D, or may be second width 22W, for example.

“During a charged period” refers to a fully charged state (SOC=100%).“During a discharged period” refers to a fully discharged state(SOC=0%). “ϵ” may be from 0.2 to 0.99, for example. “ϵ” may varydepending on the type of an active material, and/or the like. Forexample, when the active material includes graphite, “ϵ=0.7 to 0.9” maybe satisfied. For example, when the active material includes silicon,“ϵ=0.25 to 0.9” may be satisfied. For example, when the active materialincludes silicon oxide, “ϵ=0.5 to 0.9” may be satisfied. For example,when the active material includes a positive electrode active material,“ϵ=0.9 to 0.99” may be satisfied.

“U₁” and “U₂” may be derived by the following Expressions 7, 8:

U ₁=(ΔP _(los) +ΔP _(cap))d ₁ ²/(32 μL ₁)   Expression 7

U ₂=(ΔP _(los) +ΔP _(cap))d ₂ ²/(32 μL ₂)   Expression 8

where

“μ” represents the coefficient of viscosity,

“ΔP_(los)” represents straight pipe pressure loss, and

“ΔP_(cap)” represents capillary action pressure.

“ΔP_(cap)” may be derived by the following Expression 9:

ΔP _(cap)=4σ cos θ/d   Expression 9

where

“d₁” or “d₂” may be assigned into “d”,

“σ” represents surface tension, and

“θ” represents contact angle.

“λ₁” and “λ₂” may be derived by the following Expressions 10, 11;

λ₁=64 μ/(U ₁ d ₁ρ)   Expression 10

λ₂=64 μ/(U ₂ d ₂ρ)   Expression 11

where

“ρ” represents the density of the electrolyte solution.

Composition

Active material layer 20 includes an active material. In addition to theactive material, active material layer 20 may further include a binder,a conductive material, and/or the like. For example, active materiallayer 20 may be formed by applying a slurry to a surface of substrate 10in a layered manner. For example, active material layer 20 may be formedby shaping a wet powdery and granular material into a sheet form.

The active material may be in particle form, for example. The activematerial may have a D50 from 1 to 30 μm, for example. The activematerial may include a positive electrode active material, for example.The positive electrode active material is capable of occluding andreleasing lithium ions at an electric potential higher than that of thenegative electrode active material. The positive electrode activematerial may include an optional component. The positive electrodeactive material may include, for example, at least one selected from thegroup consisting of LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, Li(NiCoMn)O₂,Li(NiCoAl)O₂, and LiFePO₄. “(NiCoMn)” in “Li(NiCoMn)O₂”, for example,means that the constituents within the parentheses are collectivelyregarded as a single unit in the entire composition ratio. As long as(NiCoMn) is collectively regarded as a single unit in the entirecomposition ratio, the amounts of individual constituents are notparticularly limited. Li(NiCoMn)O₂ may includeLi(Ni_(1/3)Co_(1/3)Mn_(1/3))O₂, Li(Ni_(0.5)Co_(0.2)Mn_(0.3))O₂,Li(Ni_(0.8)Co_(0.1)Mn_(0.1))O₂, and/or the like, for example.

The active material may include a negative electrode active material,for example. The negative electrode active material is capable ofoccluding and releasing lithium ions at an electric potential lower thanthat of the positive electrode active material. The negative electrodeactive material may include an optional component. The negativeelectrode active material may include, for example, at least oneselected from the group consisting of graphite, soft carbon, hardcarbon, silicon, silicon oxide, silicon-based alloy, tin, tin oxide,tin-based alloy, and Li₄Ti₅O₁₂.

The conductive material is capable of forming an electron conductionpath. The amount of the conductive material to be used may be, forexample, from 0.1 to 10 parts by mass relative to 100 parts by mass ofthe active material. The conductive material may include an optionalcomponent. The conductive material may include, for example, at leastone selected from the group consisting of carbon black, vapor growncarbon fiber, carbon nanotube, and graphene flake.

The binder is capable of bonding the solid materials to each other. Theamount of the binder to be used may be, for example, from 0.1 to 10parts by mass relative to 100 parts by mass of the active material. Thebinder may include an optional component. The binder may include, forexample, at least one selected from the group consisting ofpolyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE),vinylidene difluoride-hexafluoropropylene copolymer (PVDF-HFP),styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyimide(PI), polyamide-imide (PAI), and polyacrylic acid (PAA).

Secondary Battery

FIG. 4 is a schematic cross-sectional view illustrating a secondarybattery according to the present embodiment.

A battery 200 includes a casing 260. Casing 260 may be hermeticallysealed. Casing 260 may have any form. Casing 260 may be a pouch made ofa metal-foil-laminated film, and/or the like, for example. Casing 260may be a metal vessel and/or the like, for example. Casing 260 may beprismatic, or may be cylindrical, for example. Casing 260 may include Aland/or the like, for example.

Casing 260 includes an electrode group 250 and an electrolyte solution(not illustrated). The electrolyte solution permeates electrode group250. Part of the electrolyte solution may remain in the bottom of casing260. Electrode group 250 may have any form. In FIG. 4 , a wound-typeelectrode group 250 is illustrated as an example. Electrode group 250may be a stack-type one, for example. Electrode group 250 includes apositive electrode 210 and a negative electrode 220. Electrode group 250may further include a separator 230. At least one of positive electrode210 and negative electrode 220 is the above-described electrode 100.That is, battery 200 includes electrode 100 and the electrolytesolution.

Separator 230 may be interposed between positive electrode 210 andnegative electrode 220. Separator 230 is electrically insulating.Separator 230 is porous. Separator 230 may be made of polyolefin and/orthe like, for example.

The electrolyte solution is a liquid electrolyte. The electrolytesolution may have a density from 500 to 2000 kg/cm³, for example. Theelectrolyte solution includes a lithium salt and a solvent. Theelectrolyte solution may further include an optional additive. Thelithium salt is dissolved in the solvent. The lithium salt may include,for example, at least one selected from the group consisting of LiPF₆,LiBF₄, and Li(FSO₂)₂N. The concentration of the lithium salt may be from0.5 to 2 mol/L, for example. The solvent may include an optionalcomponent. The solvent may include, for example, at least one selectedfrom the group consisting of ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate (BC), ethyl methyl carbonate (EMC),dimethyl carbonate (DMC), and diethyl carbonate (DEC). The additive mayinclude, for example, at least one selected from the group consisting ofvinylene carbonate (VC), vinylethylene carbonate (VEC), 1,3-propanesultone (PS), cyclohexylbenzene (CHB), tert-amylbenzene (TAB), andlithium bis(oxalato)borate (LiBOB).

EXAMPLES

Next, the present example is described.

Producing Test Electrodes

Test electrodes Nos. 1-1, 1-2, 2-1, 2-2, 2-3 were produced (see Table 1below). Each test electrode was a negative electrode. Each testelectrode included an active material layer. The active materialincluded graphite. “ϵ” was 0.9.

FIG. 5 presents top views of a groove of the test electrodes.

In the surface of the active material layer, a plurality of grooves asillustrated in FIG. 5 were formed. The groove according to Nos. 1-1, 2-1does not include a second region. The groove according to Nos. 1-1, 2-1consists of a first region. That is, the groove according to Nos. 1-1,2-1 has a uniform cross-sectional area. The groove according to Nos.1-2, 2-2, 2-3 includes a first region and a second region. Thecross-sectional area of the second region is smaller than that of thefirst region. In the present example, the cross-sectional area of eachregion was adjusted by changing the width of the region.

The dimensions and the like of each region are given in Table 1 below.Nos. 1-1, 2-1 do not include a second region, but for the sake ofconvenience, cells for “Second region Length” and the like are filledwith numerical values. For the sake of convenience, cells for “Secondregion Width” and the like for Nos. 1-1, 2-1 are filled with the samenumerical values as for the first region.

Evaluation

An electrolyte solution was prepared. The density (p) of the electrolytesolution was 1300 kg/m³. The test electrode was subjected to anelectrolyte solution permeation test. Time required for permeation ofthe electrolyte solution (permeation time) was divided by the number ofgrooves, and thereby permeation time per groove was determined.

Table 1

TABLE 1 No. 1-1 No. 1-2 No. 2-1 No. 2-2 No. 2-3 First region Length L₁(mm) 8 8 320 320 320 Second region Length L₂ (mm) 2 2 80 80 80 Firstregion Width d₁ (μm) 100 100 100 100 100 Second region Width d₂ (μm) 10050 100 50 50 First region Friction coefficient λ₁ (—) 1.23 1.23 1.231.23 1.23 Second region Friction coefficient λ₂ (—) 1.23 2.45 1.23 2.452.45 First region Average flow speed U₁ (m/s) 4.02*10⁻³ 5.02*10⁻³1.00*10⁻⁴ 1.26*10⁻⁴ 1.26*10⁻⁴ Second region Average flow speed U₂ (m/s)4.02*10⁻³ 1.00*10⁻² 1.00*10⁻⁴ 2.51*10⁻⁴ 5.02*10⁻⁴ Δ P_(con) = λ₁(L₁ +L₂)ρU₁ ²/(2d₁) (Pa) −1.29 −1.29 −0.032 −0.032 −0.032 Δ P_(exp) = λ₁(L₁ +L₂)ρU₁ ²/(2 ε d₁) (Pa) −1.43 −1.43 −0.036 −0.036 −0.036 Δ P′_(con) =λ₁L₁ρU₁ ²/(2d₁) + λ₂L₂ρU₂ ²/(2d₂) (Pa) −1.29 −8.03 −0.032 −0.200 −0.683Δ P′_(exp) = λ₁L₁ρU₁ ²/(2 ε d₁) + λ₂L₂ρU₂ ²/(2 ε d₂) (Pa) −1.43 −8.92−0.036 −2.222 −0.759 (Δ P′_(exp) − Δ P′_(con))/(Δ P_(exp) − Δ P_(con))(—) 1 6.4 1 5.5 19 Electrolyte solution permeation test (s) 2.49 1.794000 2858 2698 (Permeation time per groove) ε = 0.9 ρ = 1300 kg/m³

Results

A test electrode with a second region (No. 1-2) required less permeationtime than a test electrode without a second region (No. 1-1). It seemsthat the second region facilitated permeation of the electrolytesolution.

A test electrode with a second region (Nos. 2-2, 2-3) required lesspermeation time than a test electrode without a second region (No. 2-1).It seems that the second region facilitated permeation of theelectrolyte solution.

The greater the value “(ΔP′_(exp) −ΔP′ _(con))/(ΔP_(exp)−ΔP_(con))” is,the more reduced the permeation time tends to be.

The present embodiment and the present example are illustrative in anyrespect. The present embodiment and the present example arenon-restrictive. The technical scope of the present disclosureencompasses any modifications within the meaning and the scopeequivalent to the terms of the claims. For example, it is expected thatcertain configurations of the present embodiments and the presentexamples can be optionally combined.

What is claimed is:
 1. An electrode for a secondary battery, comprising:a substrate; and an active material layer, wherein the active materiallayer is placed on a surface of the substrate, one or more grooves areformed in a surface of the active material layer, the groove extendslinearly in a direction perpendicular to a thickness direction of theactive material layer, the groove has an open portion on a periphery ofthe active material layer, the open portion opens in the directionperpendicular to the thickness direction, the groove includes a firstregion and a second region, the second region is interposed between theopen portion and the first region, and in a cross section perpendicularto a direction in which the groove extends, the first region has a firstcross-sectional area, the second region has a second cross-sectionalarea, and the second cross-sectional area is smaller than the firstcross-sectional area.
 2. The electrode for a secondary battery accordingto claim 1, wherein in the cross section perpendicular to a direction inwhich the groove extends, the first region has a first depth, the secondregion has a second depth, and the second depth is smaller than thefirst depth.
 3. The electrode for a secondary battery according to claim1, wherein in the cross section perpendicular to a direction in whichthe groove extends, the first region has a first width, the secondregion has a second width, and the second width is smaller than thefirst width.
 4. The electrode for a secondary battery according to claim1, wherein the groove includes two second regions, and the first regionis interposed between the two second regions.
 5. The electrode for asecondary battery according to claim 1, wherein the second region isconnected to the open portion.
 6. The electrode for a secondary batteryaccording to claim 1, wherein the following Expressions 1 to 5 aresatisfied:(ΔP′ _(exp) −ΔP′ _(con))/(ΔP _(exp) −ΔP _(con))>1   Expression 1ΔP _(con)=λ₁(L ₁ +L ₂)U ²/(2d ₁)   Expression 2ΔP _(exp)=λ₁(L ₁ +L ₂)U ₁ ²/(2ϵd ₁)   Expression 3ΔP′ _(con)=λ₁ L ₁ U ₁ ²/(2d₁)+λ₂ L ₂ U ₂ ²/(2d ₂)   Expression 4ΔP′ _(exp)=λ₁ L ₁ U ₁ ²/(2ϵd ₁)+λ₂ L ₂ U ₂ ²/(2ϵd ₂)   Expression 5where λ₁ represents a pipe friction factor in the first region, λ₂represents a pipe friction factor in the second region, L₁ represents alength of the first region in the direction in which the groove extends,L₂ represents a length of the second region in the direction in whichthe groove extends, U₁ represents an average flow speed in the firstregion, U₂ represents an average flow speed in the second region, d₁represents a square root of the first cross-sectional area, d₂represents a square root of the second cross-sectional area, and ϵrepresents a ratio of a volume of the active material layer during adischarged period to a volume of the active material layer during acharged period.
 7. The electrode for a secondary battery according toclaim 6, wherein the following Expression 6 is further satisfied:(ΔP′ _(exp) −ΔP′ _(con))/(ΔP _(exp) −ΔP _(con))≥5.5   Expression
 6. 8. Asecondary battery comprising: an electrode for a secondary batteryaccording to claim 1; and an electrolyte solution.